1. Field of the Invention
The present invention relates to a method and apparatus for measuring electrical signals at circuit nodes of integrated circuits according to a sampling principle, with the assistance of a probe, particularly an electron beam probe, in which the signal is repeatedly sampled successively at different phase points.
2. Description of the Prior Art
When, for example, a status diagram of electrical signals at at least one circuit node of integrated circuits is recorded with an electron probe in accordance with a sampling method, then the sampling window is shifted across the circuit cycle in a non-linear fashion (v. U.S. Pat. No. 4,220,854 fully incorporated herein by this reference). The electrical signal is thereby cyclically applied to the measuring point. Each time the basic pulse rate of the integrated circuit has a leading edge or a trailing edge, the potential at a measuring point is usually measured by way of repeated sampling at a specific phase of the electrical signal in accordance with the prior art. Only those phase points of the electrical signal which coincide with a leading edge or with a trailing edge of the basic pulse rate of the integrated circuit are thereby acquired by this known measuring method. With this known method, the logic state changes which appear at the respective measuring point can indeed be exactly acquired because these logic state changes coincide with leading edges or with trailing edges of the basic pulse rate of the integrated circuit. When, however, brief noise pulses (spikes) occur which do not coincide with leading edges or trailing edges of the basic pulse rate of the integrated circuit, then these noise pulses are not acquired by the known measuring method. Only when noise pulses accidentally coincide with the sampling time can such noise pulses be acquired in the execution of the known measuring method.
It was heretofore necessary to record time diagrams of the electrical signals at circuit nodes of integrated circuits in order to be able to acquire spikes that do not appear at a leading edge or a trailing edge of the basic pulse rate of an integrated circuit. This means that the entire circuit cycle is linearly recorded. The electrical signal was thereby measured and recorded in the same manner at a circuit node of an integrated circuit at every phase point that is mensurationally distinguishable from another phase point. The time spent for such an overall recording of the electrical signal, however, is very high when an electrical signal at a measuring point in an integrated circuit is measured at all phase points that can be mensurationally distinguished from one another.